Arrays of photosensitive diodes are used in an assortment of applications including, but not limited to, radiation detection, optical position encoding, and low light-level imaging, such as night photography, nuclear medical imaging, photon medical imaging, multi-slice computer tomography (CT) imaging, and ballistic photon detection etc. Typically, photodiode arrays may be formed as one- or two-dimensional arrays of aligned photodiodes, or, for optical shaft encoders, a circular or semicircular arrangement of diodes.
One problem with detection devices is that they are susceptible to various radiation damage mechanisms, such as displacement damage resulting in total dose effects and ionization damage resulting in bulk effects. Both these mechanisms adversely affect the performance of detectors, transistors and integrated circuits.
Certain detector characteristics that are most affected include detector leakage current, doping characteristics, charge collection, and carrier lifetime. Over time, detectors show an increased reverse-bias current and decreased forward voltage due to radiation damage. Further, a change in doping level, due to radiation damage, adversely affects the width of the depletion region, i.e. the voltage required for full depletion and a decrease in carrier lifetime results in signal loss as carriers recombine while traversing the depletion region.
Another disadvantage with conventional detection devices is the amount and extent of crosstalk that occurs between adjacent detector structures, primarily as a result of minority carrier leakage current between diodes. The problem of crosstalk between diodes becomes even more acute as the size of the detector arrays, the size of individual detectors, the spatial resolution, and spacing of the diodes is reduced.
In certain applications, it is desirable to produce optical detectors having small lateral dimensions and spaced closely together. For example in certain medical applications, it would beneficial to increase the optical resolution of a detector array in order to permit for improved image scans, such as computer tomography scans. However, at conventional doping levels utilized for diode arrays of this type, the diffusion length of minority carriers generated by photon interaction in the semiconductor is in the range of at least many tens of microns, and such minority carriers have the potential to affect signals at diodes away from the region at which the minority carriers were generated. Therefore, the spatial resolution obtainable may be limited by diffusion of the carriers within the semiconductor itself, even if other components of the optical system are optimized and scattered light is reduced.
The conduction process in semiconductor devices is accomplished via two types of current flow: hole current and electron current. More specifically, energy can be added to electrons via external sources of energy, such as light and heat. When excess energy is absorbed by valence electrons, covalent bonds may be broken. Once the bonds are broken, the electrons move to the conduction band where they are capable of supporting electric current. When a voltage is applied to a crystal substance containing the conduction band electrons, the electrons move through the crystal toward the applied voltage, thus creating electron current flow.
In contrast, a hole results when a covalent bond is broken and a vacancy is left in the atom by the missing valence electron. The hole has a positive charge because its corresponding atom is deficient by one electron. As a result of this positive charge, a chain reaction begins when a nearby electron breaks its covalent bond to fill the hole, thus leaving another hole in an adjacent atom. This process continues, resulting in a covalent bond in the atom with the original hole and a resulting hole in the adjacent atom. Although the electron has moved from one covalent bond to another, the hole is also moving through adjacent or nearby atoms. Thus, since this conduction process results from the movement of holes rather than electrons, it is referred to as hole flow (hole current flow or conduction by holes).
Hole flow is similar to electron flow, except that the holes move toward a negative potential and in an opposite direction to that of the electrons. For example, electrons flow from negative to positive potential, whereas hole flow moves from a positive to negative potential. Majority carriers are the more abundant charge carriers, while less abundant carriers are referred to as minority carriers. In an n-type semiconductor material, electrons are the majority carriers and holes are the minority carriers. In a p-type semiconductor material, the opposite is true, thus holes are the majority carriers and electrons are the minority carriers. Since hole flow results from the breaking of covalent bonds at the valence band level, the electrons associated with this conduction must remain in the valence band. In contrast, electrons associated with electron flow, however, have conduction band energy and can move throughout the crystal.
Drift current is charged particle motion in response to an applied electric field. When an electric field is applied across a semiconductor, the carriers start moving, thus producing a current. The positively charged holes move with the electric field while the negatively charged electrons move against the electric field. The motion of each carrier is referred to as constant drift velocity, or vd, taking into consideration the collisions and setbacks each carrier has while in transit. Note that the sum total of the carriers will eventually travel in the direction they are supposed to regardless of any setbacks or collisions.
The drift current is thus the result of carrier drift, and depends upon the ability of the carriers to move around, or electron and hole mobility. The carrier concentration is another parameter that affects drift current, simply because carriers have to be present for there to be current. Yet another parameter that is measured with drift current, is the current density, which depends upon the electric field, electron or hole concentration (−/+q), the mobility constant, and the charge. Note than when a negative electric field is applied, the electrons (−q) will travel opposite the electric field, thus the resulting drift current will be positive.
Diffusion is the process of particles dispersing from regions of high concentration to regions of low concentration. If this process is left undisturbed, there will eventually be a uniform distribution of particles. Diffusion does not require external forces to act upon a group of particles. The particles move about using thermal motion. If the particles are carriers, they “carry” charge with them, thus resulting in current, and therefore diffusion current. Diffusion current will occur even when there is no electric field applied to the semiconductor. Dp, and Dn are called the diffusion coefficients, a proportionality factor. In addition, the direction of the diffusion current depends on the change in the carrier concentrations, not the concentrations themselves. Usually +q is assigned to holes whereas −q to electrons, because the carriers are diffusing from areas of high concentrations to areas of low concentrations.
Drift current and diffusion current comprise the total current in a semiconductor, although they may not be occurring simultaneously. Under equilibrium conditions, the current density should be zero because no electric field is applied. If the doping is not completely uniform, however, there will be a change in concentration in some places in the semiconductor, resulting in a gradient. This gradient may give rise to an electric field, which in turn can result in non-zero current densities.
As mentioned above, in order for current to exist, electrons and holes must move and transport charge, or exhibit mobility. Several factors can affect the mobility of a carrier, the most significant of which is scattering, or the motion-impending collisions within the crystal. These collisions can occur with an electron bumping into another electron, a hole, or even ionized impurities. Scattering may increase or decrease, depending upon factors such as temperature and/or the addition of electron acceptors/donors. In general, the higher the temperature, the more excited the carriers are, and therefore the greater the increase in scattering. The same phenomenon occurs with dopants, as not only will there be more carriers to bump into and scatter, but dopants will generate an ionized impurity that will also carry a charge. Mobility is substantially independent of the doping concentration when the doping concentration is low. The mobility of the carriers begins to decrease as the concentration of the dopant increases.
Various approaches have been used to minimize such crosstalk including, but not limited to, providing inactive photodiodes to balance the leakage current, as described in U.S. Pat. Nos. 4,904,861 and 4,998,013 to Epstein et al., the utilization of suction diodes for the removal of the slow diffusion currents to reduce the settling time of detectors to acceptable levels, as described in U.S. Pat. No. 5,408,122, and providing a gradient in doping density in the epitaxial layer, as described in U.S. Pat. No. 5,430,321 to Effelsberg.
Despite attempts to improve the overall performance characteristics of photodiode arrays and their individual diode units, within detection systems, photodiode arrays capable of reducing crosstalk while being less susceptible to radiation damage are still needed. Additionally, there is need for a semiconductor circuit and an economically feasible design and fabrication method so that it is capable of improving the spatial resolution of detectors integrated therein.